Flux and method for soldering aluminum



Nov. 14, 1961 G. J. BABCOCK ET AL 3,008,230

FLUX AND METHOD FOR SOLDERING ALUMINUM Filed April 9, 1957 400 u. r m n: 3 '2 s00 06 m n. 2 Lil 200 INVENTORS GARY J. BABCOCK DONALD C. BURCH United States Patent 3,008,230 FLUX AND METHOD FOR SOLDERING ALUMINUM Gary J. Babcock, Highland Park, Donald C. Burch,

Oak Park, and Robert L. Simpkins, Berkley, Mich.,

assignors to Essex Wire Corporation Filed Apr. 9, 1957, Ser. No. 651,756 3 Claims. (Cl. 29495) This invention relates to the art of joining metals and is concerned with a compound that is particularly useful in the soldering of metals such as aluminum and its alloys. This application is a continuation-in-part of ap plication Serial No. 343,438 filed March 19, 1953, the patentable subject matter of which was transferred to the present application before abandonment.

The soldering of aluminum has heretofore posed a twofold problem requiring seemingly opposed approaches. In the first instance, the material is coated with an extremely tenacious and tough oxide film. In order to obtain a reasonably effective soldered joint, this oxide film must be removed so that the solder metal is adjacent the base metal. To remove this tough oxide film, it is necessary that the flux have what is essentially strong corroding power. This in turn poses the second problem which is related to the corrosion resistance of the finished product. The fluxes previously available which had sufficient fluxing, or corroding power, to attack the oxide film had the unsatisfactory after effect of continued corrosion of the solder area. The soldered joints would fail in service not for want of soldering efliciency, but due to the continued after corrosion of the solder area. On the other hand, fluxes of lesser corroding power, so designed to minimize after corrosion, did not provide good aluminum fluxes because of poor fluxing of the aluminum oxide film.

The present invention provides a fluxing material which has suflicient corroding power to flux aluminum oxides. In addition, the flux is entirely consumed when heated to soldering temperatures and there is no unconsumed flux or corrosive residue remaining after the soldering operation. The corrosion resistance of various types of soldered joints has been carefully investigated. Such joints have been subjected to the salt-spray test (ASTM No. B117) and, in addition, to a humidity test wherein the atmosphere is maintained at a constant 100 percent relative humidity. In each instance, the investigation showed that the flux had been entirely consumed and that no corrosive residue remained after the soldering operation.

In addition, the present invention provides for improved soldering inasmuch as it makes possible better wetting of the solder area by the solder. This improved wetting is brought about in part by the precipitation of tin by the flux. When the flux is heated to soldering temperature, it flows freely into any interstices in the solder area, fluxing metallic oxides and depositing a thin layer of tin. Most soft solders containing tin wet more readily on the thin layer of tin precipitate with the result that joints of improved efliciency are made possible. Tension tests made on lay joints, and ring-and-plug type joints show slight variation from test specimen to test specimen indicating substantial and uniform joint etficiency. In the case of thin gauge material or small size Wire, the tin precipitate is sufiicient to complete the soldering.

The object of our invention, therefore, is to provide a new and useful flux for soldering aluminum and its alloys.

The above and other objects, advantages, and features of our invention will appear from the detailed description.

According to my invention, the compound is formed 3,008,230 Patented Nov. 14, 1961 2 by combining two salts; ammonium chloride (NH Cl) and stannous chloride (SnCl in such a manner as to form a stable, non-hygroscopic molecular combination of the two salts. The method of inducing molecular combination of the NH Cl and SnCl is quite important. Intimate mechanical mixtures of the two salts fail to produce the desired soldering or tinning characteristics.

This may be due in part to the tendency of free NH Cl to sublime rather than to liquify. In combination with SnCl however, NH C1 is made available for its fluxing power.

The two salts are combined by melting them together in the proper proportions by weight and evaporating substantially all water. Either hydrous or anhydrous SnCl may be used. The product formed is then allowed to cool and the resultant crystalline mass may then be pulverized if it is to be used as a dry flux or simply broken up if it is anticipated that it will be remelted for use.

The molecular combination of the two salts is a highly complex structure. Various proportions of the salts were prepared for development of the binary diagram shown in the figure. It can be seen from this diagram that a eutectic exists at pproximately 6 percent NH Cl and 94 percent SnCl In addition, a lower eutectic is indicated in the neighborhood of 18 percent NH Cl. The two eutectics indicate that a definite chemical compound results when the salts are melted together although the exact nature of the resultant chemical is indefinite.

The X-ray diffraction pattern of the 10 percent NH4C1 and 90 percent SnCl sample contained about lines, indicating a highly complex pattern, not identifiable from available standards.

Wet analyses indicate that the additive elements are present in unchanged proportions. This would indicate on a molecular basis that the resultant molecular balance is on the order of NH Cl:2SnCl at least at the 12 percent NH Cl level. However, as has been indicated, the molecular structure is extremely complex as indicated by the diffraction patterns.

Best fluxing action in the soldering of aluminum with the least amount of residues is accomplished with compounds corresponding approximately to the formula NH Cl2SnCl If properly applied, there are no visible residues remaining from the use of fused salt compounds containing 9 to 12. percent NH Cl and the remainder SnCl Compounds containing less than 6 percent NH CI or more than 18 percent NH CI have retarded fluxing action. Compounds containing more than approximately 18 percent by weight NH Cl have retarded and incomplete fluxing action because of insufficient stannous chloride. Those compounds containing less than 6 percent NH Cl have retarded fluxing action because of excessive residue formation.

In actual use, the present flux may be applied to any work in either a powder, as a liquid or as a slurry. The use of a powder can be simplified by heating the work slightly so that the powder softens and sticks to the work. When solder is applied to the work with the necessary heat, the flux liquifies at a reasonably low temperature and flows readily over the workpiece. The chlorides in the flux react with the oxides on the work piece to cleanse the metal, then on reaching a higher temperature, the flux reacts causing metallic molten tin to plate the Work and to alloy with it, forming a base coat to which the molten solder quickly and readily alloys forming a true solder bond. For example, it has been found that eyelet terminals to be dip soldered, need not be completely covered with flux. A small quantity may be applied to the bottom end, either from a molten flux bath or a warm terminal dipped in powder. Upon immersing this combination in molten solder, the flux melts very quickly and flows 'over the terminal and into the innermost trace of aluminum to lflpercent of aluminum to the solder. .The upper limit of aluminum as a solder additive is that amount which causes the solder to become mushy and workable at a given operating temperature. In soldering with such a combination, the chlorides in the flux combine with the aluminum in the solder (as Well as with the aluminum in the work piece, it it is used) to form aluminum chloride which passes off as a dense white cloud; this reduces the flux to metallic tin very rapidly. The tin may then plate the terminal and cable strands, or go into solution in the solder. This is an important feature in the system since it speeds up the reaction considerably; moreover, it cleans the solder surface and converts surplus flux not needed for the joint to metallic tin in the solder. The faster reaction feature would be of great value in a high volume conveyorized production setup.

-As many apparently .widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that I do not limit myself to the specific embodiments thereof except as defined in the appended claims.

I claim: 1. A flux composition for soldering aluminum consisting of the product formed by fusing together a mixture of from 9 percent by Weight NH Cl and 91 percent SnCl to 12 percent NH Cl and 88 percent SnCl 2. A flux composition for soldering aluminum consisting of the product formed by fusing together a mixture of from 9 percent by weight NH Cl and 91 percent by Weight SnCl to 12 percent by weight NH Cl and 88 percent by Weight SnCl said product consisting of compounds corresponding approximately to the formula 3. A process for soldering a member of aluminum metal comprising heating the aluminum metal member, applying a flux to the member, and applying solder to the fluxed area of the member; said flux consisting of the product formed by fusing together a mixture of from 9 percent byweight NH Cl and 91 percent by weight SnCl to 12 percent by weight NH CI and 88 percent by Weight SnCI said product consisting of compounds corresponding approximately to the formula NH Cl-2SnCl References (Iited in the file of this patent UNITED STATES PATENTS 

1. A FLUX COMPOSITION FOR SOLDERING ALUMINUM CONSISTING OF THE PRODUCT FORMED BY FUSING TOGETHER A MIXTURE OF FROM 9 PERCENT BY WEIGHT NH4CL AND 91 PERCENT SNCL2 TO 12 PERCENT NH4CL AND 88 PERCENT SNCL2.
 3. A PROCESS FOR SOLDERING A MEMBER OF ALUMINUM METAL COMPRISING HEATING THE ALUMINUM METAL MEMBER, APPLYING A FLUX TO THE MEMBER, AND APPLYING SOLDER TO THE FLUXED AREA OF THE MEMBER, SAID FLUX CONSISTING OF THE PRODUCT FORMED BY FUSING TOGETHER A MIXTURE OF FROM 9 PERCENT BY WEIGHT NH4CL AND 91 PERCENT BY WEIGHT 